Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1221 AM EST Wed Nov 28 2018

Valid Nov 28/0000 UTC thru Dec 01/1200 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

00Z NAM/GFS Evaluation...with Preliminary Preferences and
Confidence
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...Overall Pattern Across the CONUS...
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Preference: Blend of 12Z GFS / NAM
Confidence: Below Average

A strong cyclone will continue to exit into the North Atlantic
through Thursday morning while the upstream flow pattern over the
CONUS briefly goes through a lower amplitude zonal flow phase, but
with a combination of systems to reach the West Coast beginning
early Thursday which will return the flow pattern to
quasi-amplified by Friday morning. Two strong shortwave troughs,
one that ejects toward the southern Plains by the end of Day 3,
00Z December 1st, and another that approaches the CA/OR coastline
at that same time.

Across the middle of the nation, the 12Z UKMET stands out with a
stronger 700-500 mb shortwave and an outlier track with a weak
surface low into the Ohio Valley. A non 12Z UKMET blend appears
reasonable here.

The first major shortwave to reach the northern California coast
Thursday morning shows good agreement outside of the
weaker/flatter 00Z NAM. As this feature moves inland, it will
eject out into the Plains with a consensus of the
deterministic/ensemble guidance showing a negative tilt to the 500
mb trough at 00Z/01, except the NAM which is more neutrally
tilted. The 00Z GFS is not an outlier but is on the faster side of
the envelope with swinging the mid-level trough into the Plains
which affects the orientation of an inverted 700 mb trough axis
extending northwest from a central Plains 700 mb low. The 00Z GFS
orients more west-east, whereas the 12z UKMET/CMC are more
northwest to southeast. The 12Z CMC is on the fastest side of the
guidance here and not preferred given weak ensemble support.
Regarding the surface low timing/position in the Plains, there is
fairly good agreement with a non-00Z NAM blend close to the
relatively tightly clustered ensemble guidance. For now, a middle
road approach between the 00Z GFS and 12Z ECMWF is favored but
with subtle differences resulting in large differences in
impacts...hence the lowered confidence.

The second shortwave of note will reach the Pacific
Northwest-Friday night into Saturday. The differences here are
mainly in amplitude and at least through 12Z/01, a middle of the
road approach will suffice at this time lying between the 00Z GFS
and 12Z ECMWF, but with a little more weight on the deeper 00Z
GFS. The 12Z CMC is slowest with the shortwave and the 00Z NAM
favors the 00Z GFS while the 12Z UKMET favors the 12Z ECMWF.
Trends are not conclusive which also lowers confidence.

Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Otto